1. Field of the Invention
This invention is related to the field of integrated circuits and, more particularly, to the selection of threshold voltages within an integrated circuit.
2. Description of the Related Art
Due to today's advanced integrated circuit (IC) manufacturing capabilities, an IC may include millions of transistors on a single die. Depending on the application of the IC, various types of manufacturing processes may be used. For example, one common process is a complimentary metal oxide semiconductor (CMOS) process. Another process that is becoming more popular is a silicon on insulator (SOI) process. In these and other processes, the speed at which the transistors may switch from conducting (i.e., on) to non-conducting (i.e., off) may be a function of the threshold voltage (Vt) of the transistors. For example, a transistor having a low Vt may switch faster than a transistor having a higher Vt. For speed critical applications, one might be tempted to consider using only transistors having a low Vt.
However, one drawback of a low Vt transistor is the leakage current. Depending on the process, as the Vt is reduced, the leakage current increases. Although the leakage current for CMOS and SOI processes is lower than some other processes, when an IC has millions of transistors such as in microprocessors and chip multiprocessors, for example, any increased leakage currents may add up to an unacceptable level. This may be true for mobile applications where battery life may be of particular importance as well as desktop applications where the thermal budget is a consideration. Some manufacturing processes provide circuit designers the ability to select the Vt of individual transistors or groups of transistors during IC design. However, the selection process is not always straightforward nor is it always intuitive.